The invention relates to a self-focusing linear charged particle accelerator structure intended to equip a linear electron accelerator.
Linear charged particle accelerators are used in numerous fields such as scientific, medical and even industrial depending on their application, these accelerators produce beams or particles, electrons for example, having energies often ranging from one to several tens of MeV.
The electric power consumed by these accelerators is considerable, it may for example reach 130 Kw only 20 Kw of which are to be found in the accelerated beam; thus the overall efficiency of such an accelerator has a direct and considerable bearing on the cost of using this accelerator, and an improvement of its efficiency by optimizing the elements which form it is a constant preoccupation of specialists, the improvement of the efficiency being also often related to the improvement of the qualities of the beam obtained.
Linear electron accelerator structures are generally formed by a succession of resonant cavities whose dimensions are related to the frequency of an electromagnetic wave injected into the structure for accelerating the electrons, and to the speed of the electrons.
Traditionally, accelerating structures are optimized in so far as the longitudinal dynamics are concerned; the lengths of the resonant cavities which form accelerator cavities are chosen so as to accelerate the electrons continually in each of them.
The accelerating part of the electromagnetic wave is at most equal to its half period and so as to benefit from a maximum of energy yielded by this wave to the electrons, that is to say a high value of the so called "transit angle" coefficient, these cavities generally have a length l substantially equal to the product of a quarter to a third of the length .lambda.o of the electromagnetic wave multiplied by the relative speed .beta. of the electrons, in accordance with the following relationship: EQU l.apprxeq..beta.(.lambda.o/n);
where .beta. is the quotient of the average speed V of the electrons divided by the speed C of light (.beta.=V/C), and n is between 3 and 4. This length, defined within the scope of calculation of a conventional cavity, is called accelerating length.
Thus for example, in the case of an accelerator structure operating at 3000 MHz, i.e. a wave length .lambda.o equal to 100 mm and for .beta.=0.5, the accelerator cavities have a length of the order of about 12 to 16 mm, gradually increasing to reach 25 to 33 mm when .beta.=1.
This traditional approach in which optimization is limited to the longitudinal dynamics, is imperfect especially in that it does not take into account a radial defocusing effect of the beam along the accelerator structure, this effect asserting itself particularly in the first part of the structure where the energy of the electrons is still low.
This defocusing of the beam is generally compensated for by adding solenoids disposed concentrically about the accelerator structure so as to create a corrective magnetic field which increases the cost and the complexity.